Rather like guidelines, barriers are virtual views that can be used to constrain views within a layout. As with guidelines, a barrier can be vertical or horizontal and one or more views may be constrained to it (to avoid confusion, these will be referred to as constrained views). Unlike guidelines where the guideline remains at a fixed position within the layout, however, the position of a barrier is defined by a set of so called reference views. Barriers were introduced to address an issue that occurs with some frequency involving overlapping views. Consider, for example, the layout illustrated in Figure 25-12 below:
The key points to note about the above layout is that the width of View 3 is set to match constraint mode, and the left-hand edge of the view is connected to the right hand edge of View 1. As currently implemented, an increase in width of View 1 will have the desired effect of reducing the width of View 3:
Figure...